1. Field of the Invention
The present invention relates to a heat pump hot water supply system of a hot water storage type to store the hot water heated by a heat pump in a hot water tank or, in particular, to a control operation for heating up the water in the hot water tank.
2. Description of the Related Art
A conventional heat pump hot water supply system of the above type comprises a hot water tank for storing the hot water to be supplied, a circulation water path for sending the water in the lower inner part to the upper inner part of the hot water tank, a heat pump unit arranged in the circulation water path to heat the water flowing in the circulation water path to a high temperature, a high-temperature hot water path led out from the upper part of the hot water tank to supply the high-temperature hot water stored in the upper inner part of the hot water tank whenever required by an external device, a medium-temperature hot water path led out from the middle part of the hot water tank to supply the medium-temperature hot water stored between the high-temperature water in the upper inner part of the hot water tank and the water in the lower inner part of the hot water tank whenever required to an external device, a water supply path for replenishing water to the lower inner part of the hot water tank when the hot water in the hot water tank is supplied to an external device through any one of the hot water paths, a plurality of stored hot water temperature detection means arranged along the height of the hot water tank for detecting the amount and temperature of the hot water stored in the hot water tank, and a control means for controlling the operation of the aforementioned components.
In the conventional heat pump hot water supply system of hot water storage type, the heat-up control operation is performed by heating up the whole amount of the hot water in the hot water tank including the residual hot water for the preceding data to a target hot water temperature during the midnight time zone. In the last stage of the heat-up operation, therefore, residual hot water higher in temperature than that of water is supplied to and heated in the heat pump unit. However, there is caused a problem in this heat pump hot water supply system, wherein the lower the temperature of the water supplied to a condenser for heating the water in a refrigeration cycle (heat pump cycle), the higher an operating efficiency (COP). Therefore, with the increase in the temperature of the water supplied to the condenser representing most of the residual medium-temperature hot water, the operating efficiency of the heat pump cycle is undesirably reduced.